Been meaning to ask this for a while and now finally have a proper sub-forum to ask the question. I'm planning on doing a barge board / flat floor / rear spat / vented front fender design at some point relatively soon. Hoping I could get some advice to guide my design and learn a thing or two.

Is there a method to the madness on designing DIY barge boards without any means of CFD? It would seem to me that you are trying to control airflow without understanding the airflow as its difficult to visualize.

I currently have front wheel spats where the outer edge of the spat is flush with the wheel face to cover up the front area of the tire. Does designing a barge board on the other side of the tire to come out just as far make sense?

What is the drawback to designing a barge board that sticks out too far or not far enough? Drag vs downforce like a splitter?

With the airflow behind the front wheel being disrupted, is 6mm alumalite still strong enough to use for the application or should 10mm be used like what's recommended for splitters? 6mm is a bit cheaper and lighter which would be preferred if possible.

I'm sure i'll have more questions to ask as the thread develops, but just wanted to get a start to get the creative juices flowing.

There are two primary focuses; 1) vent all of the pressure out of the wheel well area and 2) prevent the travel of airflow that is moving down the side of the car from wrapping under the car

1 is all about reducing front end lift in the most basic form and in more comprehensive designs, other aero devices or heat exchanges may vent into the front wheel well and so venting the front wheel well serves an even greater purpose.
2 is a lift and drag reduction technique. Generally, airflow under the car is moving a bit faster than the air beside the car thanks to less obtrusion, and the faster that air under the car moves via smoothing the under-body etc., the greater the pressure differential between the air beside the car and the air under it. Thus, if left unimpeded the air will follow that pressure differential and wrap under the doors and increase lift and drag as it increases turbulence, hits the rear tire, etc.. The Miata's body shape along the side is particularly conducive to this negative effect. It's beneficial for any car to reduce this, but of utmost importance for cars with significant under-car aero as in those cases it's crucial to isolate the airflow feeding the under-car elements. Preventing that under-car flow can be achieved with a vertical skirt, but I prefer the barge board approach because it provides a surface area for the pressure differential to act on and create a bit of downforce.

Yes, 6mm alumalite will do the job here, that's what I use. Here is my current setup, with a new version of the front fenders to be coming sometime next year:

There's certainly a point of development and complexity past which CFD is necessary to ensure you're doing any good. DTM/JGTC cars (they're merging in 2020) provide a perfect example of fender and barge board areas that are developed via extensive simulation. This level of complexity would be pointless without CFD (Photo is a 2017 GT500 GTR):

For contrast, and appropriately timed since WTAC is just a day away, I would submit the HKS TRB-03. This was also designed via substantial simulation, but (in the area that this discussion relates to) relies on basic principles to maximum effect. This could be replicated via DIY methods with good results:

Can someone explain why a flat floor works "without" using a skirt type flap on the outer edges between the wheels (sorry i cant think of a word for this)....

I would think a flat floor and no skirt (or edge) would encourage air from the side coming in to the underbody flow and that adding a vertical edge (skirt) would discourage that.....for ref see motis car versus Supe's rx7.

edit:: and common belief (currently) is that reducing or limiting side air to underfloor is a bad thing.

I am interested in this for 2 reasons,
- Like Ryan, a flat floor between axles is not allowed in any class we run but flat sides with/without side skirts likely are (grey area).

- The Video Chris Harris did on the Porsche 962 where the designer, Norbert Singer said that unlike F1 of the time, the tunnels on the 962 worked BETTER with air coming in from the sides than without. Specifically at 2.45 - 3.00 in the following video.

Maybe the difference is flat floor versus ground effects tunnels...but I would think it would be similar in theory, especially if your running a rear diffuser. So it seems like opposite arguments.

The answer is flow control and pressure differential.
Comparing a prototype to a formula car is a bit like apples and oranges, especially without having the full contemporary specs of both classes.
What you want is to control the air flow in order to maintain / improve the pressure differential between the top and bottom.
As a rule of thumb for the Miata shape (if there ever was such thing..), the air would love to get under the car along the sides, and thus reducing your pressure diff and losing potential downforce.
Setting a barrier along the way is "a good thing".

The answer is flow control and pressure differential.
Comparing a prototype to a formula car is a bit like apples and oranges, especially without having the full contemporary specs of both classes.
What you want is to control the air flow in order to maintain / improve the pressure differential between the top and bottom.
As a rule of thumb for the Miata shape (if there ever was such thing..), the air would love to get under the car along the sides, and thus reducing your pressure diff and losing potential downforce.
Setting a barrier along the way is "a good thing".

I appreciate that, I just thought it weird and the opposite of what is "common understanding" that they would test the conventional way with skirts (as in F1 of the time) only to find that air from the sides improved the downforce...

Anyway, as per your comments above, is their a reason you didn't add skirts to your flat floor between the wheels to reduce the air ingress from the sides?

I am unsure of the reason as I am not Moti but the article on Morpheus by MotoIQ says that splitter creates vortices down the side sealing the ingress of air.
How much sealing is actually happening Moti would need to clarify but another thing is a skirt would be rubbed away making it a replaceable part which is not that desirable.
I always wondered if a thin wall of bristles would do the trick.

The horizontal extension of the flat bottom IS the barrier in my case.
Running lower you start risking damage, at the end of the day this is a Miata that deals with production car compromises, not a purpose design Porsche 962 with springs that have variable wire diameter and coil wind.
Ride height -

Keep in mind that the pic above was taken with the car running 1400/700 spring rates, it's not soft.

The horizontal extension of the flat bottom IS the barrier in my case.
Running lower you start risking damage, at the end of the day this is a Miata that deals with production car compromises, not a purpose design Porsche 962 with springs that have variable wire diameter and coil wind.

Keep in mind that the pic above was taken with the car running 1400/700 spring rates, it's not soft.

Yeah, driving with so much spring is an interesting experience!
Choosing the rate was an educated guess I had to take before SLB because Creampuff was set to run with more downforce than any other Miata.
Emilio and Ryan were both running a softer setup but have since gone up to rates that are in the same neighborhood.